1. Field of the Invention
The present invention relates to a broadcast receiver, and more particularly, to a symbol timing recovery for minimizing a dispersion constant using received data.
2. Discussion of the Related Art
FIG. 1 is a block diagram of a general digital receiver. Referring to FIG. 1, the digital receiver includes an antenna 101, a tuner 102, an analog processor 103, an analog/digital (A/D) converter 104, a phase separator 105, a carrier recovery 106, a symbol timing recovery 107, a channel equalizer 108, a phase tracer 109, a forward error correction (FEC) decoder 110, and an audio/video (A/V) signal processor 111.
In operation, a sky-wave signal received through the antenna 101 is converted into a pass-band signal of an intermediate frequency (IF) by the tuner 102. Subsequently, the converted signal passes through the analog processor 103 consisting of a surface acoustic wave (SAW) filter, an automatic gain controller (AGC), for removal of inter-channel interferences and high frequency components generated from the tuner 102.
The A/D converter converts an analog signal into a digital signal. At this time, if a fixing oscillator is used in the A/D converter 104, an analog signal is converted into a digital signal having a fixed frequency.
The pass-band signal converted into a digital signal at the A/D converter 104 passes through the phase separator 105 and is converted into a complex signal. The complex signal passes through the carrier recovery 106 and is converted into a baseband signal.
The signal lowered down to the baseband through the carrier recovery 106 is inputted to the symbol timing recovery 107 for recovering a symbol clock.
At this time, a reception party should generate the same clock as the clock that has been used upon transmission so as to recover received data. Such a function is performed by the symbol timing recovery 107.
The symbol timing recovery 107 is positioned in a baseband of the digital television (TV) receiver and delivers synchronized symbol data to the channel equalizer at the rear end.
As described above, a signal from the symbol timing recovery 107 is inputted to the channel equalizer 108 and the channel equalizer 108 removes inter-symbol interferences added by a transmission channel. A residual phase of a carrier, not removed by the carrier recovery 106 is corrected by the phase tracer 109.
The signal whose phase is corrected in this manner passes through the FEC decoder 110 where an error generated while passing through a channel is corrected and the signal passing through the FEC decoder 110 is delivered to the A/V signal processor 111.
The A/V signal processor 111 decrypts video and voice signals processed in a moving picture experts group-2 (MPEG-2) and a Dolby audio coding-3 (AC-3) type and allows the signals to be outputted through a monitor and a speaker 200.
A basic construction of the symbol timing recovery 107 among elements of such a digital broadcast receiver is illustrated in FIG. 2.
As illustrated in FIG. 2, the symbol timing recovery 107 includes a resampler 201, a timing error detector 202, a loop filter 203, and numerically Controlled Oscillator (NCO) 204.
In operation, a signal A/D-converted by the A/D converter 104 passes through the phase separator 105 and the carrier recovery 106 and inputted to the resampler 201 of the symbol timing recovery 107. The resampler 201 generates an intermediate value of a signal sampled at the A/D converter 104, thereby outputting a sample whose phase is close to a phase of an actual symbol timing frequency.
Subsequently, an output from the resampler 201 is delivered to a timing error detector 202, where a timing error is extracted by various algorithms. A timing error signal outputted from the timing error detector 202 passes through the loop filter 203 so that a low-band signal component is filtered. A DC output signal of the loop filter 203 is inputted to the NCO 204, which provides a sampling clock depending on the inputted DC.
FIG. 3 is a block diagram of a Gardner-type symbol timing recovery widely adopted in fields of a digital TV (DTV) receiver and digital communication.
Operation of the Gardner-type symbol timing recovery will be described with reference to FIG. 3. First, a complex signal from the carrier recovery is inputted to a resampler 301, which generates a sample having twice larger frequency than the symbol frequency.
A prefilter 302 passes only a real part in a signal outputted from the resampler 301 and prefilters one half point of the symbol frequency to reduce a pattern jitter due to data.
The signal prefiltered by the prefilter 302 is inputted to a Gardner timing error detector 303 (Gardner TED), which detects a timing error from an inputted signal and outputs the timing error to a loop filter 304 at a rear end.
After that, the loop filter 304 passes only a low band signal component in information regarding a timing error inputted from the Gardner TED 303 and outputs the low band signal component to a numerically controlled oscillator (NCO) 305.
The NCO 305 converts an output frequency depending on the low band component of the timing error to generate a control signal for controlling a sampling timing of the resampler 301.
An output of the resampler 301 under control of the control signal generated at the NCO 305 is outputted to the channel equalizer.
Generally, for the symbol timing recovery, it is required that a fast synchronization capture be performed and the symbol frequency be traced with minimum noise after convergence.
For the fast synchronization capture for the big timing offset, an average gain (i.e., S-curve) of the timing error detector should be large and a convergence characteristic of a timing recovery loop should be good.
Particularly, for a fast synchronization capture even for a ghost close to 0 dB, an average gain characteristic of the timing error detector is very important.
As is well known, a convergence characteristic of the Gardner type timing error detector depends on a gain of an upper band edge of a spectrum positioned at a point of one half a symbol frequency. If the upper band edge of the spectrum is seriously faded in a frequency selective channel circumstance, the symbol timing recovery cannot converge, thus the whole system performance is deteriorated.
That is, as illustrated in FIGS. 4A and 4B, an average gain (S-curve) of the timing error detector reaches almost zero for 1 symbol delay 0 degree (phase) 0 dB ghost or 2 symbol delay 180 degree 0 dB ghost where a null is generated in a spectrum that corresponds to one half of the symbol frequency. As described above, if a null is generated in a relevant data edge, the symbol timing recovery has a problem of not being able to capture the timing offset at all.
FIGS. 5A and 5B are graphs illustrating simulation results of a convergence characteristic of the timing recovery loop by ignoring influence of jitter by carrier recovery and forcibly giving an initial timing offset that corresponds to about 0.0001 times the sampling frequency.
Examination of FIGS. 5A and 5B reveals that in case there exists 1 symbol delay 0 degree (phase) 0 dB ghost or 2 symbol delay 180 degree 0 dB ghost where a null is generated, the symbol timing recovery does not converge to the initial timing offset value. Such results are represented because the average gain characteristics (S-curve) of the timing error detector is reflected as it is, by which a problem that the symbol timing recovery cannot capture the timing offset is confirmed again.
To partially compensate for such disadvantages of the Gardner type, a method for normalizing an output of the timing error detector 303 or a method for adjusting a gain is used. Such methods can reduce a convergence time but increases a jitter component after convergence under influence of a noise amplification, thus dose not provide a fundamental solution for a case where a symbol frequency component is seriously faded.